Dry Cask 101 – Radiation Shielding

We’ve talked before about how the uranium in nuclear fuel undergoes fission during reactor operations. The fission process turns uranium into a number of other elements, many of which are radioactive. These elements continue to produce large amounts of radiation even when the fuel is no longer supporting a chain reaction in the reactor. So shielding is necessary to block this radiation, and protect workers and the public.

As we discussed in an earlier blog post, the four major types of radiation differ in mass, energy, and how deeply they penetrate people and objects. Alpha radiation—particles consisting of two protons and two neutrons—are the heaviest type. Beta particles—free electrons—have a small mass and a negative charge. Neither alpha nor beta particles will move outside the fuel itself.

But spent fuel also emits neutron radiation (particles from the nucleus that have no charge) and gamma radiation (a type of electromagnetic ray that carries a lot of energy). Both neutron and gamma radiation are highly penetrating and require shielding.

Shielding is a key function that dry storage casks perform, but the two main types of dry storage casks are configured in slightly different ways.

For welded, canister-based systems, shielding is provided by a thick (three feet or more) steel-reinforced concrete vault that surrounds an inner steel canister. The thick concrete shields both neutron and gamma radiation, and may be oriented either as an upright cylinder or a horizontal building.

In bolted cask systems, there is no inner canister. Bolted casks have thick steel shells, sometimes with several inches of lead gamma shielding inside. They have a neutron shield on the outside consisting of low-density plastic material, typically mixed with boron to absorb neutrons.

The NRC reviews spent fuel dry cask storage designs to ensure they meet our limits on radiation doses beyond the site boundary, under normal and accident conditions, and that dose rates in general are kept as low as possible. Every applicant must provide a radiation shielding analysis as part of the application for a new storage system, or an amendment to an existing system. This analysis uses a computer model to simulate radiation penetration through the fuel and thick shielding materials under normal operating and accident conditions.

We review the applicant’s analysis to ensure it has identified all the important radiation-shielding parameters. We make sure they’re modeled conservatively, in a way that maximizes radiation sources and external dose rates. We may create our own computer simulation to confirm the dose rates provided in the application. That helps us to ensure the design meets off-site radiation dose rate requirements under all conditions.

Radiation monitoring is required by NRC regulations and storage system certificates of compliance for all spent fuel storage systems. This includes measuring both gamma and neutron radiation at various points around each spent fuel storage cask, including the vents in the concrete overpacks for canister-based storage systems. The amount of radiation coming through the vents varies with the storage cask design, and the fuel stored in the canister, but all canister-based systems are designed to minimize the radiation dose rate at the vents. Additionally, the total gamma and neutron dose equivalent rate from all sources at the site, including the spent fuel storage systems and the reactor itself, must be less than 0.25 milli-Sieverts (25 millirem) per year at the controlled area boundary.

Tritium comes primarily from neutron activation of water, while carbon-14 comes primarily from neutron activation of nitrogen. These products, if present at all, would only be found in trace amounts in the dry environment inside a storage cask. But more importantly, the casks are not leaking at all, so they would not be leaking tritium or carbon-14.

You are correct that higher temperatures correspond to greater amounts of fission inside the cask. Because casks are designed to remain sub-critical, this fission could not become self-sustaining as it is in an operating reactor. Most of the heat in a spent fuel cask actually comes from alpha-decay of transuranic isotopes such as plutonium and americium.

I never knew that there were methods like shielding to prevent workers from coming into contact with radioactive elements. I wish they would touch on this more in school. Even in my college chemistry classes, where we learned about gamma and alpha radiation, we never learned that things like concrete can be used in factories to protect people who must be in contact with it.

The neutron source from spent nuclear fuel is from either alpha-neutron reactions or spontaneous fission of heavy isotopes. Irradiation of uranium in a nuclear reactor will produce elements heavier than uranium due to successive absorption of neutrons. Isotopes of these heavier elements, as well as remaining uranium isotopes, typically decay by alpha particle emission. Alpha particles will react with certain other isotopes to produce neutrons, in a reaction known as an “alpha-n” reaction. Heavier elements may also have isotopes that decay by spontaneous fission, where fission occurs without absorbing a neutron. Although these heavy isotopes are not present in concentrations sufficient to cause criticality, spontaneous fission will produce two to three neutrons per reaction, and is the primary source of neutrons for most spent fuel discharged today. Section 5.1 of NUREG/CR-6700, “Nuclide Importance to Criticality Safety, Decay Heating, and Source Terms Related to Transport and Interim Storage of High-Burnup LWR Fuel,” provides more information about relative importance of these decay processes to the neutron source.

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